Qualification of Laser Welded High Performance S44660 Ferritic Stainless Steel for Condenser Tube Applications

The laser tube welding process has for the first time been used to produce power plant steam condenser tubing in the high performance ferritic stainless steel grade UNS S44660. This material has traditionally been produced as welded tubing by the gas tungsten-arc welding process for use in seawater and other severe cooling water environments. To verify the corrosion resistance of this new product, corrosion tests have been conducted on production tubing to compare the traditional and new welding processes. The acidified ferric chloride test was used for evaluation because it is a meaningful aggressive test capable of measuring resistance to localized pitting corrosion, the most common potential failure mode for stainless steels used in cooling water environments. Pitting tests conducted over a range of temperatures produced a critical pitting temperature of 65°C for laser welded-annealed tube. This critical pitting temperature was demonstrated to be equal to that of as-produced S44660 sheet material or that of gas tungsten-arc produced tubing. The tubing met all other metallurgical and mechanical property quality requirements. When pitting did occur it exhibited no preference for initiation at welds. Thus, laser welded. high performance stainless condenser tubing should be fully capable of providing good performance in severe cooling water environments.